In a promising experiment in an affluent swath of Brooklyn, New York, dozens of solar-panel arrays spread across rowhouse rooftops are wired into a growing network. Called the Brooklyn Microgrid, the project is signing up residents and businesses to a virtual trading platform that will allow solar-energy producers to sell excess-electricity credits from their systems to buyers in the group, who may live as close as next door. The project is still in its early stages — it has just 50 participants thus far — but its implications could be far reaching. The idea is to create a kind of virtual, peer-to-peer energy trading system built on blockchain, the database technology that underlies cryptocurrencies like Bitcoin.